1. Field of the Invention
The present invention relates generally to an anti-skid brake control system for an automotive vehicle which can precisely control a skid control cycle for optimizing vehicular braking performance. More specifically, the invention relates to noise elimination in derivation of wheel acceleration as an important parameter for precisely controlling a skid cycle in an anti-skid brake control system.
2. Description of the Background Art
There have been conventionally proposed various anti-skid brake control systems. For example, Japanese Patent Second (examined) Publication (Tokko) Showa 51-6305 disclose for an anti-skid brake control system. In the disclosed system, anti-skid control is generally initiated in response to a negative wheel acceleration which is a wheel deceleration hereafter simply referred to as wheel acceleration including positive values and negative values, for avoiding confusion by reference to both acceleration and deceleration, decreasing across a predetermined wheel deceleration threshold. The anti-skid control is performed over one or more skid control cycles in which braking pressure in a wheel cylinder is controlled according to a predetermined schedule for maintaining the braking pressure close to lock pressure so as to optimize braking performance. In most cases, the wheel deceleration threshold is set in view of desired maximum wheel deceleration on a high friction road. This causes overshooting in braking pressure control because of the presence of lag time in detection of wheel acceleration. Such overshooting tends to cause degradation of braking performance by leading the wheel into locking due to excessive braking pressure.
In order to solve this problem in the prior art, Japanese Patent Second (examined) Publication (Tokko) Showa 53-14708 discloses an anti-skid brake control system which is provided mutually different wheel deceleration thresholds to be selectively used according to the friction level of the road surface. Though such an approach may reduce the magnitude of overshooting and thus reduce the possibility or the magnitude of wheel locking for improving vehicular braking performance, it still leaves various problems unsolved. For instance the aforementioned prior proposed system does not account for the influence of noise superimposed on wheel speed data as base data for deriving the wheel acceleration. Therefore, avoidance of the influence of noise in the derivation of wheel acceleration for improving the level of precision in anti-skid control is necessary.